Britain runs a secret internet-monitoring station in the Middle East to intercept and process vast quantities of emails, telephone calls and web traffic on behalf of Western intelligence agencies, The Independent has learnt.

The station is able to tap into and extract data from the underwater fibre-optic cables passing through the region.

The information is then processed for intelligence and passed to GCHQ in Cheltenham and shared with the National Security Agency (NSA) in the United States. The Government claims the station is a key element in the West’s “war on terror” and provides a vital “early warning” system for potential attacks around the world.

The Independent is not revealing the precise location of the station but information on its activities was contained in the leaked documents obtained from the NSA by Edward Snowden. The Guardian newspaper’s reporting on these documents in recent months has sparked a dispute with the Government, with GCHQ security experts overseeing the destruction of hard drives containing the data.

The Middle East installation is regarded as particularly valuable by the British and Americans because it can access submarine cables passing through the region. All of the messages and data passed back and forth on the cables is copied into giant computer storage “buffers” and then sifted for data of special interest.

Information about the project was contained in 50,000 GCHQ documents that Mr Snowden downloaded during 2012. Many of them came from an internal Wikipedia-style information site called GC-Wiki. Unlike the public Wikipedia, GCHQ’s wiki was generally classified Top Secret or above.

The disclosure comes as the Metropolitan Police announced it was launching a terrorism investigation into material found on the computer of David Miranda, the Brazilian partner of The Guardian journalist Glenn Greenwald – who is at the centre of the Snowden controversy.

Scotland Yard said material examined so far from the computer of Mr Miranda was “highly sensitive”, the disclosure of which “could put lives at risk”.

The Independent understands that The Guardianagreed to the Government’s request not to publish any material contained in the Snowden documents that could damage national security.